It is well known in the surface acoustic wave art that resonators pose a particular problem in their construction. Resonators require a constant velocity throughout their structure and typically include first and second reflective gratings with first and second spaced transducers inserted between the gratings. If the transducers are very close to each other, electromagnetic cross-talk occurs because the transducers are very capacitive. Thus, they must be separated or isolated from each other electrically. When they are separated, an unmetalized region occurs between the two transducers. That region does not have the same velocity as the metalized regions, those having the transducer electrodes or the reflective gratings thereon. This means that the unmetalized region or cavity is no longer resonant at the same frequency as the gratings or the transducer. Therefore, the cavity must be changed in length to perturb the frequency. This is a very complicated process but must be accomplished since the cavity, without electrodes, causes an insertion loss and distortion of the phase response of the resonator.
Thus, a center grating must be added to cause a constant velocity through the region separating the two transducers. Further, it is often desired that the center gratings do not cause reflections. If .lambda./4 electrodes are used as the coupling grating, the velocity of the acoustic waves through the cavity is constant, but reflections occur from the center grating electrodes. Split-finger electrodes, well known in the art, could be used as a center grating and no reflections would occur, but then the velocity through the region would be different because there are a different number of edges per given length. Thus, with .lambda./4 electrodes, there are four edges for the two electrodes and a 50/50 metallization or 50% of the region is metalized and 50% is free space. If split-finger electrodes are used, there is still a 50/50 metallization to free space ratio, but there are now eight reflector edges instead of four. The velocity of the acoustic wave is affected by energy stored at the electrode edges and the split-finger electrode has twice as many edges. Thus, the velocity through the split-finger electrodes is different than the velocity through a structure having .lambda./4 electrodes.
Further, it is also known in such resonators that the resonators are most generally constructed with uniformly distributed reflectors. For example, all electrodes may be .lambda./4 in width and separated by .lambda./4 free space regions. The reflection characteristic of a uniform reflector has relatively strong side lobes. In order to reduce this disadvantage, withdrawal weighted reflector gratings are used. Withdrawal weighting is the selective omission, or withdrawing, of reflective elements or electrodes. Proper withdrawal weighting causes reduced reflection side lobes. However, as soon as some of the electrode fingers or elements are removed, the velocity is changed through the grating, thus creating distortion and insertion loss. In order to compensate for these disadvantages in the prior art, the remaining electrodes have to be shifted or moved to a different position on the substrate to compensate for the missing electrodes. In order to determine where the electrodes must be positioned, one has to know the acoustic velocity on the free area compared to what it is on the metalized surface and the calculations become very complicated and precise placement of the electrodes is almost impossible. Further, the electrodes are no longer on equally-spaced grids because they vary in position nonlinearly and thus it is impossible to make such a mask with an E-beam. E-beam systems are well known in the art and basically include an electron beam to write the desired pattern on a photoresist. The E-beam operates digitally on a grid system. Further, with each different type of metal that is used or with a change in metal thickness, new calculations have to be performed because of the different velocities relating to the different metals or different thicknesses and thus a separate mask must be designed for each different type of metal or metal thickness used.
In commonly assigned copending application Serial No. 608,354, filed Nov. 2, 1990, and incorporated herein in its entirety by reference, there is disclosed a group-type structure with 3/8.lambda. and 5/8.lambda. sampling. Reflectionless transducers and broadband notch elements are all implementable with those new configurations. Although they are reflectionless, they do not have the same velocity as a structure having electrodes with a width of .lambda./4 and separated by a gap of .lambda./4. This structure is generally known as a two-electrode-per-wavelength structure. In commonly assigned copending application Ser. No. 510,964, filed Apr. 19, 1990, and incorporated herein by reference in its entirety, an electrode structure is disclosed which has no reflections and has the velocity of a uniform two-electrode-per-wavelength structure. One disadvantage of this structure is that it has substantially lower coupling than the uniform two-electrode-per-wavelength structure. Further, because of the very narrow electrode width in the structures, 1/8.lambda. the electrodes tend to have a resistance loss. Thus, it has two disadvantages first it has lower coupling than a uniform two-electrode-per-wavelength structure and second it has higher resistance because of the narrow 1/8.lambda. lines.
The present invention has the advantages of both the group-type coupling and the constant velocity of a uniform two-electrode-per-wavelength structure. Thus, there is higher coupling and lower resistance.
To construct such a surface acoustic wave structure having no reflections and the velocity of a uniform two-electrode-per-wavelength structure, multiple electrodes are placed on a piezoelectric substrate with only four spaced electrodes for each 2.lambda. distance of structure. The electrodes are all one-quarter wavelength and are separated from each other by a space of .lambda./8. A space of 5/16.lambda. on each end of the group completes a 2.lambda. distance. Thus in an electrode distance of 2.lambda., the structure has fifty percent metallization, it has the same metal thickness everywhere and the same number of edges as the uniform two-electrode-per-wavelength structure and it has no reflectivity. These groups repeat as necessary along the electrode structure. Because the group of electrodes consists of electrodes having a width of .lambda./4 and separated by a gap of .lambda./8, the structure inherently cancels reflections among all four electrodes so that no reflections occur outside the group. Thus, a large number of groups would have no reflections in the entire structure because of the reflectionless nature of each group of electrodes. The 5/8.lambda. gap that separates adjacent groups simply keeps the transduction in phase from one group to the next group. Thus, the special configuration of the electrodes and spaces in each group give zero reflectivity within each group while the 5/8.lambda. space separating adjacent groups keeps the transduction in phase between groups. This structure, then, has the advantages of the group-type structure and the advantages of the zero reflectivity structures as set forth in the copending applications described earlier.
Again, the structure has constant velocity because, over the entire electrode structure, only two electrodes per wavelength are used. Thus, it is acoustically equivalent to a uniform .lambda./4 structure with two electrodes per wavelength. It has a metal-to-free-space ratio of 50/50 and it has the same number of electrode edges as a structure having two electrodes per wavelength. Thus, the velocity through the structure is constant. It has no reflections because the four electrodes in any group when considered together cancel the reflections from each other because of the particular spacing and width of electrodes used. Therefore, this structure has the equivalent velocity performance of a two-electrode-per-wavelength structure, but it has no reflectivity as does the two-electrode-per-wavelength structure. This structure can be used in either gratings or transducers to cause nonreflective gratings or nonreflective transducers but both of which have constant velocity throughout the structure.
Where it is desirable to have a two electrode per wavelength transducer or grating with variable reflectivity, such as the case with external gratings where it may be desired, for example, to have a tapered reflectivity to reduce side lobes and insertion loss, the structure having two electrodes per wavelength is arranged such that 2.lambda. distances of electrodes in the two-electrode-per-wavelength structure are replaced with the groups of electrodes set forth in the present invention. These inserted structures will have the same velocity as the two-electrode-per-wavelength grating or transducer but will have no reflections. Thus, the grating or transducer can be withdrawal weighted as desired to change the reflectivity of the grating or transducer as a whole. By combining the nonreflective structure disclosed herein and the reflective structures of the prior art, greater control over the amount of reflectivity of a transducer or grating structure can be controlled to obtain any desired tapering of the reflectivity.
Thus, an important aspect of the present invention is to provide an electrode structure that has the same velocity as a two-electrode-per-wavelength structure but has substantially no reflectivity.
It is another aspect of the present invention to provide a group of only four spaced electrodes on a substrate for at least one 2.lambda. distance of electrode structure with each of the electrodes having a width of substantially .lambda./4 and a center-to-center spacing of substantially 3/8.lambda. between adjacent ones of the four spaced electrodes such that the electrode structure has the velocity of the uniform two-electrode-per-wavelength structure and no reflections.
It is still another aspect of the present invention to provide withdrawal weighting of an electrode structure without changing the velocity through the structure.